Custom Merge

Hi everyone!

In my project, I have a lot of image files, that I wanted to process using a different tool. For this, after reading some posts about custom merge drivers in this forum, I make the following modifications:

In the file .gitconfig I added the following lines:

[merge "imgfile"]
        name = Driver for image files
        driver = /usr/local/bin/git-merge-img.sh %O %A %B %L
        recursive = binary

and in the .gitinfoattributes I put:

*.png   merge=imgfile

As I can see, it works calling my custom script during a merge between
 two branches that have a .png file. Unfortunately, even after a successfully 
merge, the final image is not merged with the one in the other branch. 
In order to test it, I've called my script using three images, giving me the 
corrected result.

I don't know if it could help but instead of changing the content of the file
 I create another one with the same name during a merge operation.

Does anyone could help me pointing what could be wrong with this?

Thanks a lot!